But at that instant, another sensation swept through me as quick as lightning. I was conscious of the presence of the Count, and of his being as if lapped in a storm of fury. As my eyes opened involuntarily I saw his strong hand grasp the slender neck of the fair woman and with giant's power draw it back, the blue eyes transformed with fury, the white teeth champing with rage, and the fair cheeks blazing red with passion.
So this is really interesting. Jonathan seemed to almost feel Dracula's presence before he ever says anything. There's no mention of the sound of a door or window either; it's like he just appears in the room the same way the vampire ladies later vanish, passing through the window perhaps. And yet he has a sort of energy that suffuses the room. Jonathan made the comparison to a storm - maybe something like how you can almost feel the pressure change in the air before a storm.
Dracula has a presence too. Maybe it's always there but gets more intense when he is angry or actively exerting his magical abilities in some way. I just... think it's a really cool detail.